Paging amplifiers are often connected to a PABX or other system signal source wherein a large number of amplifiers are driven in common with announcement, music, etc. signals. Such amplifiers may be built into wall boxes or may be packaged with or drive speakers attached to walls or within ceilings.
While the problem of driving the amplifiers by a common signal has long been satisfactorily solved, the problem of powering the amplifiers has not. The paging amplifiers are typically powered from a secure paging system main power supply, in order that emergency announcements would not be disrupted in the case of a utility power failure.
The paging amplifiers are sometimes powered via a common 70 volt line. However such a line has exhibited induction problems if it were placed too close to telephone lines which very often are run parallel to the paging line.
In order to substantially eliminate the induction problem, local paging amplifiers are sometimes connected in phantom with telephone lines. However the telephone wires are only narrow gauge (e.g. #22 wire), and therefore only a limited number of paging amplifiers could be used, driven from a limited length of line since voltage drop in the telephone wires limits the amount of current that can be provided to the paging amplifiers. Therefore a separate pair of heavy gauge wires is often separately run to carry power to each of the paging amplifiers. Typically a pair of relatively heavy gauge, e.g. #18 gauge power wires would extend to each paging amplifier, as well as relatively light #22 gauge telephone wires carrying the paging signal. This type of system could accommodate a large number, e.g. hundreds, of paging amplifiers without the induction problems of the 70 volt line, but suffers from the high installation and wire cost of the extra heavy gauge pair of wires.